In a liquid chromatograph (LC), a variety of detectors, such as an absorbance detector, fluorescence detector, deferential refractive index detector, and electric conductivity detector, are used. Such detectors have an analog-to-digital (A/D) converter for sampling an analog detection signal at predetermined data sampling intervals and converting the sampled analog values into digital values, and send the digitized detection signal to a personal computer or other devices to perform a data processing task (refer to Patent Document 1 and other documents). The sampling period in the A/D converter is usually determined in view of the following factors: a short sampling period in the A/D converter increases the temporal resolution; however, it also increases the amount of data generated per unit time, increasing the required amount of memory as well as the load on the communication system and the central processing unit (CPU). On the other hand, a long sampling period decreases the temporal resolution and steep peaks cannot be reproducibly detected, although it decreases the load on the communication system and CPU. Considering such advantages and disadvantages, the sampling period is set to be an appropriate value or range, e.g. approximately 1 through 100 [msec].
Generally, the signal for determining the timing of the data sampling in the A/D converter is generated by counting a radio-frequency reference clock signal created using a crystal oscillator. Hence, the frequency deviation of the crystal oscillator mainly causes the lag between the progress of real time (which is based on the International Atomic Time) and that of the time inside the detector (which will be hereinafter called an “apparatus internal time”) which accompanies the repetition of the data sampling. Since the retention time of a peak on the chromatogram generated based on the detection signal is based on the apparatus internal time, the lag between the apparatus internal time and real time appears as a lag of the retention time on the chromatograph.
The maximum frequency deviation of generally available crystal oscillators is approximately ±100 through 200 [ppm]. If the frequency is deviated to a higher value, the apparatus internal time progresses faster than real time. Therefore, the retention time of a peak on a chromatograph becomes longer than it really is in accordance with the amount of the lag. For example, assuming that the frequency deviation of a crystal oscillator is +150 [ppm], the time lag after 100 minutes from the initiation of an analysis is 0.015 minutes, which is practically negligible. However, after 2000 minutes from the initiation of the analysis, the time lag increases to 0.3 minutes. This lag can be easily recognized on the chromatogram by a user, and is not negligible.
In the case where an eluate whose components are separated in a column is bifurcated and the bifurcated eluates are respectively and simultaneously detected by two detectors, each of the detectors has its own A/D converter which samples data in accordance with a different reference clock signal. Therefore, the difference of the frequency deviations of the two reference clock signals, i.e. those of the crystal oscillators for generating a reference clock signal, emerges as the difference of the retention times on the chromatogram obtained in each detector.
FIG. 6A illustrates an example of an output screen (only a chromatogram display screen) shown after a predetermined time has passed from the initiation of an analysis in a conventional LC system. In this example, the chromatogram data and other data were initiated to be plotted at the time when 2000 minutes had elapsed from the initiation of the analysis, and FIG. 6A illustrates the plots around the time when 4019 minutes (or 2019 minutes from the initiation of the plotting) elapsed. The position of the “4019 minutes” in real time is denoted by an outline arrow in the figure; however, the data of the fluorescence detector B have further progressed to the position t1, which is approximately 0.3 minutes longer than real time. This corresponds to +150 [ppm] of the frequency deviation of a crystal oscillator. On the other hand, the data of the fluorescence detector A have only progressed to the position t2, which is before the “4019 minutes” and approximately 0.08 minutes shorter than real time. This corresponds to −40 [ppm] of the frequency deviation of a crystal oscillator. The data of the cell temperature and the column oven temperature on the screen are updated, based on the monitoring information sent from a unit which is different from the detector, in correspondence to a readout request from a personal computer. Their lag against real time is as small as 0.015 minutes.
An actual chromatographic analysis does not generally require such a long time as described above. However, for example, if an analysis operator commands the initiation of an analysis before going home on Friday to perform the analysis during the night and weekend and comes to work on Monday morning to view the result, the output screen shows such a state as FIG. 6A. Since this output screen obviously shows that the apparatus internal time is not accurate, even if this does not cause substantial problems, the analysis operator doubts the accuracy of the detector, which might damage his or her credibility of it. In particular, the longer the elapsed time becomes from the initiation of an analysis, the more the difference becomes between real time and the apparatus internal time, and the aforementioned problem becomes more prominent.
Recently, the advance of the technique of chromatographic analysis has made it possible to obtain very steep peaks, requiring a higher temporal accuracy than before for the retention time of such steep peaks. Nevertheless, there is also a more practical problem in that such an inaccuracy of the apparatus internal time as described earlier makes it difficult to reduce the error of the retention time.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H06-174709